1. Field of the Invention
The present invention is in the field of producing controlled dopings in side walls and at the bottom of trenches etched into semiconductor substrates by diffusing a dopant out of a layer which has been deposited in the trenches and which contains the dopant.
2. Description of the Prior Art
Doped trenches of the type with which the present invention is concerned form the basis of capacitor storage cells (trench cells) which, because of their small space requirements, are used in highly integrated semiconductor circuits as DRAM stores (dynamic random access memory) in order to gain additional space.
The trench cells require a high level of doping which has previously been produced by p-trough technology in which ions of the dopant, accelerated in an electric field, are implanted into zones of the semiconductor substrate by a masking technique.
A method of producing a phosphorus doping in the walls and at the bottom of trench cells is described, for example, in an article by Morie et al appearing in IEEE, 1983, pages 411 to 414. In this disclosure, a phosphorus silicate glass (PSG) is deposited in the trenches which have been etched into the substrate by the chemical deposition of phosphine (PH.sub.3) and silane (SiH.sub.4) in an oxygen atmosphere from which the phosphorus diffuses into the substrate at high temperatures.
These processes have a number of disadvantages. An additional masking step is required for the p-trough technique. In order to attain deep trough depths, a high temperature process is necessary, lasting for many hours, and which nevertheless leads to a dopant concentration which diminishes considerably in the direction of the trench base. A similarly unfavorable concentration gradient occurs when PSG layers are diffused in accordance with the above described process due to the fact that the PSG layer becomes thinner with increasing trench depth. This gradient reduces the maximum capacitance attainable in the trench cell and leads to a high degree of sensitivity to leakage currents.